I don't recall ever seeing a study but I do recall something about it being brought up a long time ago.

Applicable now is that copper is really expensive in comparison. Manufacturing a copper rad is always going to cost more than aluminum just due to cost of materials.

What I recall a long time ago when copper pricing was negligible was the manufacturing process of attaching the fins to the tubing. With aluminum you were able to directly secure them to each other without using an unlike metal. With copper manufacturing the fins had to be attached with a non-copper solder which mean heat was actually going through water to the copper piping -> to a non-copper material -> to copper fins which actually dampens the potential performance. This may no longer be the case with modern manufacturing.

Not sure how valid that is today. I do recall the overall emissive performance of identical aluminum vs copper rads to be pretty negligable considering the now large price difference that can occur with material costs